Apple powdery mildew, caused by Podosphaera leucotricha is a major problem for growers of apple trees. Apple powdery mildew is present in all apple producing areas of the world. Apple powdery mildew causes symptoms on young shoots, leaves, blossoms and fruit. The fungus reduces tree vigor, flower bud production and fruit quality.
There are generally two phases to apple powdery mildew infection. The fungus overwinters in dormant flower and shoot buds that were infected the previous year. As the temperatures warm, the fungus and buds break dormancy and the fungus infects the new growth. Conidia are released from the new growth and the spores cause secondary infections where they land.
Currently, it is very difficult to control apple powdery mildew for several reasons. First, Podosphaera leucotricha has developed resistance to some fungicides. Secondly, some effective fungicides are being banned in some areas because they are harmful to the environment or humans.
Yet another issue is that often the currently available treatments have to be applied repeatedly to be effective and to treat secondary infections. Some apple orchards are treated with fungicides for apple powdery mildew up to 12 times in a growing season. The extra applications add to the expense of the treatment because more of the product must be purchased, and significant time and labor resources are used during the subsequent applications. Further, each new application increases the risk of exposure of the product to non-target trees.
Other means of treating trees are available but have proven to be ineffective or impractical to implement. Some methods require professionals who are trained in specialized equipment to apply the product. Other expensive methods include invasive drilling, bark injections, or high-pressure root flare injections. For example, there was a fungicide that provided some systemic fungal protection, however, it was effective only when injected directly into the tree.
Non-chemical treatments such as waxes and oils have also been used, but they provide inconsistent results. Manually removing the infected material from trees is extremely expensive and not practical for large orchards.
Therefore, there is a need in the art for safe and effective methods for providing long-term protection to apple trees from apple powdery mildew infection.